IRVING — Dallas Cowboys owner Jerry Jones was optimistic enough after a loss in Baltimore to say his team could evolve into a Super Bowl contender “not next year — this year.”

His tune and tone changed a bit when the Cowboys lost star linebacker Sean Lee to a season-ending toe injury this week. He all but declared the team’s leading tackler irreplaceable.

“That’s a setback,” Jones said. “The old adage that it gives someone else a chance to show what they’ve got doesn’t necessarily apply here because he’s a proven difference-maker out there.”

Jones said all this with the Cowboys facing one of their biggest games of the year on Sunday. The NFC East-leading Giants will be in town, boasting the NFL’s second-rated offense and eager to avenge a season-opening loss to Dallas.

The Cowboys also have to play the second straight game without their best running back, DeMarco Murray. The team isn’t saying when Murray will return from a foot injury sustained in the Baltimore game two weeks ago, although Jones said he didn’t think it would be long term.

“I’ve said all along I think this team has an opportunity to be a contender, but the one exception that I made was ... sitting as we’re sitting right now health-wise, injury can make a difference and does in the NFL,” said Jones, whose 90-year-old mother, Arminta Jones, died this week.

Lee sustained ligament damage in his right big toe in last weekend’s 19-14 win at Carolina, and surgery is scheduled for next week. He said he will be sidelined three to four months but should be healthy by February — in time for offseason workouts.

Coach Jason Garrett and his teammates expect Lee to be an active participant in practice and on game day, and the third-year pro out of Penn State indicated as much Thursday.

“You have to be positive because if you’re negative you’re not going to get any better,” said Lee, who missed a year in college with a knee injury and sat out part of last season with a dislocated wrist before finishing the season in a cast. “As frustrating as it is, there’s no getting around it.”

Bruce Carter will take over Lee’s spot and the defensive play-calling for the Cowboys, while Dan Connor, Lee’s close friend and former Penn State teammate, will slide into Carter’s spot. Connor signed as a free agent after four years at Carolina.

“I think everybody is going to miss him,” Carter said. “He really took the job serious. That’s something I’ve got to step up as a player for myself. I don’t want to let those other guys down.”